Read The Zombie Survival Guide by Max Brooks. Based on the observations in this book, the AA-12 would be terrible for fighting a zombie invasion. Low ammo, short range, and shhots too fast. And ammo is bulky too. remember: zombies can't just be shot and die. Either destroy the brain with a headshot, or completely destroy the zombie. A shotgun would cause massive damage, but at the range it takes to blow a head off, you would not want to be that close to a zombie horde. Especially if you need to reload oftem. Use a semi-auto rifle, even a .22, over a shotgun for zombies, and go for headshots only. As for killing humans, this AA-12 rig would be a death machine.

blowing the entire head off is not a necessity. as implied by the use of a .22 bullet, you would need to only damage the brain with even just one BB from a buckshot, if it can pierce the skull and penetrate into the brain, it will be enough to disrupt the functionality of the brain, without actually removing all brain matter from the skull. so with multiple zombies in close proximity of each other, a shotgun would be efficient, so long as your using some good buckshot rounds.

This is actually part of my point, though I worded it badly. A .22 is effective because it goes through the skull once, then bonces around on the inside, destroying the brain. Were it to go straight through at a higher velocity, it would cause less damage, not able to kill a person, and possibly not a zombie.